The World Health Organization has designated the highly mutated coronavirus strain detected in South Africa as a "variant of concern". The UN health body has also given it a name: Omicron.
The WHO made the announcement at an emergency meeting on Friday. A statement says preliminary evidence suggests an increased risk of reinfection compared with other variants of concern.
Health authorities in South Africa announced on Thursday that 77 cases had been detected in Gauteng Province, which includes the country's largest city, Johannesburg.
The variant has also been found in Botswana, Hong Kong, Israel and Belgium.
The WHO's technical lead on COVID-19, Maria Van Kerkhove, says the variant has nearly 100 mutations. She says this can affect how the virus behaves.
She also says it will take weeks before scientists can determine the level of protection offered by current vaccines.